In this type of window assembly, it is usual to replace the air in the space between the glass sheets with a gas to improve the insulation of the double glazed unit. To achieve this, one or two holes are made in the spacing unit.
One such sealing device for double glazed window assembly is described in U.S. Pat. No. 5,345,734 issued Sep. 13, 1994 to Applicant. In this sealing device, a flexible plug is provided to fit into holes made in the spacing unit to allow the gas to be injected into the space between the sheets. A blind rivet is used with the plug. An outward pull is exerted on the rivet causing an expansion of the plug to thereby sealingly contain the gas injected between the glass sheets. With this sealing method, the enlarged head portion of the stem of the rivet remains in the sealing device so that no further access is provided to test or to obtain information as to the condition of the gas present in the space between the two glass sheets.